Gas etching followed by gas plating



Aug. 12, 1969 E slRTL GAS ETCHING FOLLOWED BY GAS PLATING Filed Feb. 1, 1966 United States Patent :2 J Int. Cl. B4411 1/18, 1/34; C23c 11/00 US. Cl. 1172ll3 12 Claims ABSTRACT OF THE DISCLQSURE Described is an improvement in the method of producing thin crystalline layers of high-purity material by a reversible chemical transport reaction of a gas mixture between a source and a substrate in a reaction vessel, comprising maintaining a temperature gradient between source and substrate and controlling the transport reaction to reverse the transport direction for selectively removing and depositing material at the substrate. The improvement comprises enforcing the selective reversal of the transport direction by changing the total pressure of the reaction gas mixture in the reaction vessel, and simultaneously maintaining a substantially constant molar mixing ratio of the gas mixture and a constant temperature range of the reaction. For example, with silicon as the high purity material, iodine as the reaction gas and a temperature between about 1050 C. and about 1250 C., the direction of transport is reversed by changing the total gas pressure from below to above a critical value between about 15 torr and about 85 torr.

My invention relates to a method for producing thin crystalline, preferably monocrystalline layers of highly pure materials upon a heated substrate by means of a chemical transport reaction.

Such a production of thin layers, especially monocrystalline films on monocrystalline substrates, requires the substrate surface to be free of crystalline defects. Mechanically polished substrates possess a surface layer greatly damaged by the action of the polishing material. This damage layer must be removed in most cases. Doing this by chemical etching or polishing to the extent required for attaining a satisfactory surface which is sufiiciently planar and exhibits only slight roughness, involves numerous disadvantages. The use of liquid etchants may introduce contamination. The gas etching process as heretofore known is troublesome because it results in nonuniform elimination of surface material particularly noticeable with thin substrate wafers or discs.

It has been proposed to purify the substrate surfaces prior to deposition of crystalline material, by subjecting the surface to a chemical transport reaction obtained with the aid of a component which is added to the reaction gas and effects the desired erosion. It has been further proposed to adjust the temperature gradient between substrate and source material in such a manner that material will be removed from the substrate surface. Another attempt prescribed temporarily limiting the reaction space to thereby adjust the equilibrium condition in such a manner as to first eifect elimination of material from the substrate surface, and thereafter removing the spacial limitation of the reaction space to cause precipitation of semiconductor material onto the substrate surface.

It is an object of my invention to devise a process which affords a selective and readily controllable elimination and precipitation of material relative to the substrate in a particularly simple and reliable manner, as compared with the methods heretofore known.

According to the invention, a reversible transportation ice of material along the direction of a temperature gradient is effected by changing the total pressure of the reaction gas mixture in the reaction vessel while simultaneously maintaining constrainedly a constant molar mixing ratio and a constant temperature range.

The invention is predicated upon the recognition that a reversal in transportation of material takes place at a given critical pressure peculiar to the particular chemical transport reaction system being used. This recognition is utilized, for example, by adjusting the total pressure of the reaction gas mixture to a value which, for eliminating material from the substrate surface, is below the critical value peculiar to the transport reaction system, and then maintaining this subcritical pressure value until the desired amount of material is removed from the substrate surface. Thereafter, the growing of a layer on top of the substrate is commenced by changing the total pressure of the reaction gas mixture to a different value above the critical value of the particular transport system, and thereafter maintaining this supercritical pressure constant until the desired layer thickness is attained.

The process may be performed by effecting the reversal in transport direction only once, so that first an amount of material is removed from the substrate surface and thereafter an epitaxial layer is grown upon the surface. However, the reversal in direction of material transportation may also be repeated. This is particularly advantageous if layers of different semiconductor materials or of the same semiconductor material but with respectively different dopants or dopant concentrations, are to be produced.

The transport reaction may be performed, in accordance with the sandwich method, between two closely juxtaposed crystal plates or discs of which one constitutes the source material and the other the substrate for the epitaxial layer to be grown.

In an analogous manner the method of the invention is applicable for transport reactions in which the source material and the substrate are spaced a relatively large distance from each other.

A great variety of materials are applicable with the method of the invention. Suitable for example, are silicon in the system silicon-iodine, titanium in the system titanium-iodine, zirconium in the systems zirconium-chlorine, zirconium-bromine and zirconium-iodine, also vanadium in the system vanadium-iodine, niobium in the systems niobium-chlorine, niobium-bromine, tantalum in the systems tantalum-chlorine, tantalum-bromine and copper in the system Cu OHCl.

The values of the critical pressure specific to the chosen transport reaction system can be calculated from the equilibrium constants as well as from the formation enthalpy of the individual reaction partners. For example, the critical pressure for the transport of silicon in the iodine system is 15 torr at 1050 C. 35 torr at 1150 C., and torr at 1250" C. For comparison, the transport of titanium in the iodine system at 1250 C. becomes reversible at a critical pressure of 20 torr.

The adjustment of the critical pressure can be made by providing for a corresponding negative pressure in the reaction vessel, or by admixing inert gas to the halogen vapor or helagenide vapor which forms part of the reaction gas.

Suitable as substrates for the layers to be precipitated are inert materials such as quartz, carbon (pyrographite), silicon carbide, highly refractory oxides. Further applicable are tungsten, tantalum or other metals of a sufliciently high melting point to remain stable at the reaction temperature. For producing monocrystalline layers, the substrates may consist of the material to be precipitated or of materials having the same crystalline lattice structure and approximately the same lattice constant. The crystalline constitution of the precipitated layers largely depends upon the constitution of the substrate material.

Layers suitable for use in semiconductor devices may be produced by adding doping materials to the source material or to the reaction gas in the conventional manner.

The method according to the invention is favorably applicable for producing semiconductor circuit components such as transistors, rectifiers and the like, as well as for the production of symmetrically conducting circuit components such as ohmic resistors.

For further elucidating the invention, reference will be had to the embodiments of processing equipment for performing the method, illustrated by way of example on the accompanying drawing in which:

FIG. 1 shows schematically and in section an apparatus for performing the transport reaction by a sandwich method between two closely adjacent discs of crystalline material; and

FIG. 2 shows schematically and in section an apparatus for converting a polycrystalline material of low purity to a high-purity monocrystalline product.

The apparatus according to FIG. 1 comprises a reaction vessel 1 of quartz having a gas inlet and an outlet with respective valves 11 and 12. Mounted inside the vessel is an electric heater 2 whose terminals 3 are located outside of the reaction vessel for connection to a voltage source. The heater may be formed of silicon-coated graphite, for example. Placed on top of the heater 2 is a monocrystalline disc 4 of silicon to serve as source material. A substrate disc 5 on top of the source disc 4 also consists of monocrystalline silicon. The apparatus serves to produce a chemical transport reaction which removes material from the source disc 4 and precipitates it upon the adjacent surface of the substrate 5. An optimal distance between the disc 4 and 5 is secured by a spacer 6 of quartz or silicon. The reaction gas, containing iodine vapor, enters the reaction vessel 1 in the direction of the arrow 7 through the valve 11. In the reaction space the iodine reacts with the silicon in accordance with the equation The reaction gas leaves the vessel 1 through the outlet 8 and the valve 12, this being indicated by an arrow 9. For adjusting the gas pressure in the reaction vessel, the outlet 8 is connected with a vacuum pump which is not illustrated. A vacuum meter 10 is provided for measuring the total pressure inside the reaction vessel.

When using the apparatus, the source disc 4 is heated by means of the heater 2 to a temperature of about 1250 C. The substrate 5, being heated by direct heat transfer from the source 4, is at an about 50 C. lower temperature than the source material. After these operating temperatures are reached and kept constant, the pressure in the reaction vessel is first set to a value of about torr. At this gas pressure there occurs an elimination of material from the substrate 5. Thus the surface of the substrate is cleaned from any impurities and liberated from any surface damage so that a particularly smooth surface will result. Thereafter the pressure in the reaction vessel is increased to about 150 torr. This causes the transport direction to reverse so that now semiconductor material from the more highly heated source plate 4 is eliminated and precipitates onto the surface of the substrate 5 where a thin monocrystalline layer is grown.

If desired, the source disc 4 and the substrate 5 may differ from each other with respect to doping. For example, a substrate 5 of p-type silicon may be provided with an epitaxial layer of n-type silicon, the source disc 4 in this case being n-type material.

However, the source disc and the substrate may also be chosen of material having the same type of conductance and either the same or respectively different electrical conductance values. Furthermore, doping material may be added to the reaction gas serving as the transporting medium. In this case, the crystalline discs 4 and 5 are preferably made of materials having the same conductivity (specific resistance).

If several layers of respectively different conductivity and different types of conductance are to be precipitated upon each other, the precipitation of material onto the substrate may be interrupted for a short interval of time, for example between the precipitation of the individual layers. Such temporary interruption may be effected by reducing the total gas pressure in the reaction vessel below the critical value of about torr. At this pressure the transport of material from the source disc 4 to the substrate 5 will cease. By then varying the concentration of the doping material or by adding respectively different doping materials to the reaction gas, layers of respectively different conductivity are produced. The thickness of the individual layers can then be adjusted by correspondingly selecting the duration of the precipitating process.

If it is desired to operate in a closed reaction system rather than with a continuous flow of reaction gas, it is preferable to heat a source 14 of halogen or halogenide to a different temperature. The source 14 can be closed by means of a valve 13. In some cases it is necessary to also heat the other vessel walls in order to prevent condensation of the vapors at undesired localities.

The apparatus represented in FIG. 2 is particularly advantageous if a material of a relatively low purity, available for example in polycrystalline form, is to be precipitated in hyperpure and monocrystalline form. For example, thin layers of titanium can be produced in this manner. The transportation of the titanium is likewise effected in the iodine system, using Til as reaction gas for example. The gaseous TiI reacts with the titanium and forms Til; in accordance with the equation At a temperature of about 1250 C., there first occurs a removal of titanium under formation of Til the total pressure of the reaction gas in the reaction vessel being kept below a critical value of 20 torr, for example at 10 torr. Thereafter, while maintaining the same reaction temperature, the pressure in the reaction vessel is increased to a value above approximately 50 torr. Now Til is dissociated into Til and Ti, and the titanium simultaneously precipitates in solid constitution. Depending upon the choice of the substrate material, the precipitated layer grows in monocrystalline or polycrystalline form.

The reaction just described is readily performed in apparatus as shown in FIG. 2 and described presently. Used as reaction vessel is an elongated tube 21 of quartz having a gas inlet 22 at one end and a gas outlet 23 at the opposite end. The tube is surrounded by two electric furnaces 24 and 25. For adjusting a temperature gradient in the reaction vessel, the furnace 24 and 25 are adjusted to respectively different temperatures. A single furnace may also be used if it permits maintaining an adjustable temperature gradient. The reaction gas, for example vaporous TiI is supplied in the direction of the arrow 26 through a valve 27. The residual gases are withdrawn through the outlet 23 and a valve 28 in the direction of the arrow 29 by means of a vacuum pump (not illustrated). The valves 27 and 28 permit closing and sealing the reaction vessel. The gas pressure in the reaction vessel is measured by a manometer 30.

Lumps 31 of raw titanium, used as source material, are heated by furnace 24 to a temperature of about 1250 C. The substrate 32 is maintained by means of the furnace 25 at a temperature about 40 C. lower than that of the source material.

During the first stage of the process, the pressure of the reaction gas (T11 is adjusted to about 5 to 10 torr. At this gas pressure there occurs an elimination of material from the substrate 32 by transport-reaction etching. When the desired amount is eliminated, the gas pressure in the reaction vessel is raised to above the critical pressure of about 20 torr (at 1250 C.), so that now the source material (raw titanium) 31 is converted to gaseous TiI At the colder substrate 32 there occurs a dissociation of the TiI from which metallic titanium precipitates upon the substrate. Depending upon whether the substrate is polycrystalline or monocrystalline, the precipitating titanium forms a grown thin layer of crystalline or monocrystalline constitution.

If with this arrangement the reaction is to be performed in a closed system rather than with a flowing medium, the apparatus is to be equipped with an additional source of halogen or halogenide corresponding to the source 14 shown in FIG. 1.

The above-described process can be repeated several times so that the purity degree of the resulting titanium can be greatly increased in this manner. Aside from titanium and silicon, various other materials can be processed by an analogous transport reaction, for example zirconium, vanadium, niobium, tantalum or copper. In each case, the constitution of the precipitated layers greatly depends upon the crystalline constitution of the substrate being employed. That is, a monocrystalline substrate as a rule is required if the precipitated layer is to grow as a monocrystal.

I claim:

1. In the method of producing thin crystalline layers of high-purity material selected from silicon, titanium, zirconium, vanadium, niobium, and tantalum, by a reversible chemical transport reaction, by producing a reaction gas through the action of a halogen containing transport gas upon a heated source and supplying the reaction gas via a temperature gradient to the substrate whereby the surface of the substrate body is first subjected to a gas etching by means of the transport gas, the transition from gas etching to precipitation is then brought about by reversing the transport direction, said selective reversal of the transport direction being achieved by changing the total pressure of the reaction gas mixture in the reaction vessel, and simultaneously maintaining a substantially constant molar mixing ratio of the gas mixture and a constant temperature range of the reaction.

2. The method according to claim 1, which CO1 prises setting the total gas pressure in said vessel, for removal of material from the substrate, to a value below the critical pressure of the chemical transport system being used, and maintaining the gas pressure at said subcritical value until the desired amount of material is removed from the substrate.

3. The method according to claim 1, which comprises setting the total gas pressure in said vessel, for deposition of material on the substrate, to a value above the critical pressure of the chemical transport system being used, and maintaining the gas pressure at said supercritical value until the desired amount of material is deposited on the substrate.

4. The method according to claim 1, wherein said substrate is a monocrystal, and which comprises setting the total gas pressure in said vessel to a value below the critical pressure of the chemical transport system being used, maintaining the gas pressure at said subcritical value until a damage layer is removed from said substrate, and thereafter changing the total gas pressure to a supercritical value for growing an epitaxial layer on the substrate.

5. The method according to claim 1, which comprises repeating the reversal of the transport direction.

6. The method according to claim 1, which comprises placing two crystal plates in parallel juxtaposition and performing the transport reaction in the interspace, one of said plates forming said source, and the other plate forming said substrate.

7. The method according to claim 1, which comprises spacing the source from the substrate in said vessel, and separately heating source and substrate to respectively different temperatures.

8. The method according to claim 1, wherein the material of said source is silicon and said reaction gas is essentially formed of iodine.

9. The method according to claim 1, wherein the material of said source is titanium and said reaction gas is essentially iodine.

19. The method according to claim 1, wherein said substrate is a silicon monocrystal, the source is formed by silicon and the reaction gas is essentially iodine, and which comprises heating the source to a constant temperature between about 1050 C. to about 1250 C. while maintaining a lower temperature at the substrate, and reversing the direction of transport by changing the total gas pressure from below to above a critical value between about 15 torr and about torr.

11. The method according to claim 10, which comprises heating and maintaining the silicon source plate at about 1250 C. and the substrate at about 1000 C., setting the total gas pressure to a low value which is less than the critical value of 85 torr to etch the substrate surface, and thereafter setting the total gas pressure to a high value above 85 torr for growing an epitaxial layer on the substrate.

12. The method according to claim 11, wherein said low pressure value is about 20 torr and said high value is about torr.

References Cited UNITED STATES PATENTS 3,168,422 2/1965 Allegretti et al. 3,316,130 4/1967 Dash et al 148-175 ALFRED L. LEAVIIT, Primary Examiner A. GOLIAN, Assistant Examiner US. Cl. X.R. 

